This invention relates to an investment casting method that utilizes rapid cooling during solidification to achieve a desired uniform fine microstructure for an as-cast component.
Investment casting is typically used to produce parts having complex shapes that are cost prohibitive to produce by other casting methods or which cannot be made by other methods such as sand or permanent mold casting. During the investment casting process, molten metal is poured into a pre-heated ceramic shell mold. A known characteristic of the ceramic shell mold used in investment casting is slow solidification. Slow solidification produces a coarse and heterogeneous casting microstructure. When conventional aluminum alloys are cast, this coarse and heterogeneous microstructure is acceptable because the microstructure for the final product can be altered to a desired microstructure by post-casting heat treatment.
Slow solidification becomes disadvantageous when the cast alloy needs to be used in an as-cast form and fast cooling rates are required to achieve the desired uniform fine microstructure. This is particularly true when dispersion strengthened alloys are processed via casting methods. For example, when cast high temperature aluminum alloys are used in an as-cast state, or with minimal post-casting heat treatment to produce aircraft engine parts, rapid cooling during solidification is a crucial part of achieving the desired as-cast microstructure.
Thus, it is desirable to provide an improved investment casting process that accommodates fast cooling of the casting during solidification while using conventional investment casting ceramic shell molds. It is also desirable for the improved investment casting process to be easily incorporated into existing foundry facilities in addition to overcoming the above referenced deficiencies.
The subject invention provides an investment casting process that permits rapid cooling of the casting while using conventional ceramic shell molds. Molten material is poured into the pre-heated ceramic shell mold. The mold is then rapid cooled by quenching the shell mold in an oil bath.
In the preferred embodiment, the molten material is a high temperature aluminum alloy having a melting point temperature approximately between 600 to 700xc2x0 C. Prior to quenching the mold in the oil bath, the molten material is preferably maintained at a temperature approximately between 50 to 100xc2x0 C. above the melting point temperature. The oil preferably has a flash point greater than the melting point temperature of the aluminum alloy and has a low viscosity at room temperature.
The cooling rate can be specifically tailored to various component types/shapes by controlling/varying the immersion rate of the shell mold into the oil bath. Cooling rate can also be controlled by varying the type of oil, e.g., oils having different flash points and viscosities, or by varying the temperature of the oil. Cooling rate is also a function of the thickness and permeability of the shell mold.
The subject invention provides an improved investment casting process that utilizes rapid cooling during solidification to achieve a desired final as-cast microstructure. These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.